Dry Tree Semi-Submersible With Reduced-Height Drilling Floor

ABSTRACT

A floating, semi-submersible offshore drilling vessel has a blowout preventer located on the semi-submersible instead of on the seabed. The mud return housing is attached directly to the blowout preventer without the use of a slip joint. This configuration allows for a significantly reduced drill floor height and, consequently, a lower vertical center of gravity for the rig.

CROSS-REFERENCE TO RELATED APPLICATIONS:

This application claims the benefit of U.S. Provisional Application No. 61/638,291 filed on Apr. 25, 2012.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to oil and gas well drilling. More particularly, it relates to floating offshore drilling rigs.

2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98

A variety of floating vessels can be used for offshore drilling and production operations. Semi-submersible vessels (“semis”) are a particularly popular vessel for drilling operations. A semi-submersible is a floating unit with its deck supported by columns to enable the unit to become almost transparent for waves and provide favorable motion behavior. The unit stays on location using dynamic positioning and/or is anchored by means of catenary mooring lines terminating in piles or anchors. A Semi-submersible Drilling Rig is fitted with drilling facilities and dynamic positioning system for deep-water drilling.

“Mud” is a term that is generally synonymous with drilling fluid and that encompasses most fluids used in hydrocarbon drilling operations, especially fluids that contain significant amounts of suspended solids, emulsified water or oil. Mud includes all types of water-base, oil-base and synthetic-base drilling fluids. Drill-in, completion and work over fluids are sometimes called muds, although a fluid that is essentially free of solids is not strictly considered mud.

Drilling fluid(s) comprise any of a number of liquid and gaseous fluids and mixtures of fluids and solids (as solid suspensions, mixtures and emulsions of liquids, gases and solids) used in operations to drill boreholes into the earth. Drilling fluid is synonymous with “drilling mud” in general usage, although some prefer to reserve the term “drilling fluid” for more sophisticated and well-defined “muds.”

In a typical drilling operation, mud is pumped down the drill string and returns to the surface carrying the drill cuttings via the annulus between the drill string and the well bore (or casing). The mud is returned to a shale shaker—the primary and probably most important device on the rig for removing drilled solids from the mud. This vibrating sieve is simple in concept, but a bit more complicated to use efficiently. A wire-cloth screen vibrates while the drilling fluid flows on top of it. The liquid phase of the mud and solids smaller than the wire mesh pass through the screen, while larger solids are retained on the screen and eventually fall off the back of the device and are discarded. Obviously, smaller openings in the screen clean more solids from the whole mud, but there is a corresponding decrease in flow rate per unit area of wire cloth. Hence, the drilling crew may seek to run the screens (as the wire cloth is called), as fine as possible, without dumping whole mud off the back of the shaker. Where it was once common for drilling rigs to have only one or two shale shakers, modern high-efficiency rigs are often fitted with four or more shakers, thus giving more area of wire cloth to use, and giving the crew the flexibility to run increasingly fine screens.

The “Christmas tree” (or just “tree”) in drilling operations is the set of valves, spools and fittings connected to the top of a well to direct and control the flow of formation fluids from the well. In offshore drilling operations, the tree may be either “wet”—i.e., located on the seabed or “dry”—i.e., located above the surface on the drilling vessel.

A Deep-Draft Semi® drilling vessel with a dry tree operating in the Gulf of Mexico may be subjected to severe heave motions during a major hurricane or winter storm conditions. This can pose a challenge in the design of the associated drilling riser tensioner as well as the well-bay and drilling structure arrangement. A key to successful development of a dry tree semisubmersible is to balance the conflicting design requirements of stability and in-service motion.

Due to the large heave motion of a dry-tree semi, the blowout preventer installed at the surface on a drilling riser above the drilling riser tensioner will have a large range of vertical motion relative to the semi in a major hurricane. With the drilling riser tensioner installed on the semi lower deck, the drill floor needs to be located high enough above the semi lower deck to accommodate the large range of estimated blowout preventer motions in a major hurricane. Consequently, the higher the drill floor the higher the vertical center of gravity of the drilling structure, which increases the moment arm of the wind load. Hence, it poses challenges to the platform stability and overall design.

A typical drilling riser slip joint (or “travel joint”) provides one solution which accommodates the vertical motion of the dry-tree semi relative to the surface stationary blowout preventer. A slip joint is a telescoping joint at the surface in floating offshore operations that permits vessel heave (vertical motion) while maintaining a riser pipe to the seafloor. As the vessel heaves, the slip joint telescopes in or out by the same amount so that the riser below the slip joint is relatively unaffected by vessel motion. A slip joint may be installed between the diverter, located below the drill floor well center, and the top of the blowout preventer, to complete the mud system to direct returning mud back to the shakers/mud pits. A slip joint that can accommodate 35 ft. of vertical movement or stroke, estimated during a major hurricane, will have a minimum closed length of 48-50 ft. Allowing for a full 35 feet of stroke requires 65.5-67.5 ft. of open area between the blowout preventer and the drill floor.

One way of reducing the drill floor height is for operational procedures to stipulate the removal of the slip joint in case of a major hurricane. The drill floor can be lowered enough to use a shorter slip joint that would still be needed to accommodate heave of up to 15 ft. expected in a hurricane originating in the Gulf of Mexico or normal winter storms. This requires the emergency abandonment procedures to allow sufficient time to remove the slip joint before a major hurricane. Moreover, slip joint removal is an operational step that always involves health, safety, security and environmental risks.

U.S. Pat. No. 6,913,092 describes a system for the return of drilling fluid from a sealed marine riser to a floating drilling rig while drilling. A floating rig or structure for drilling in the floor of an ocean using a rotatable tubular includes a seal housing having a rotatable seal connected above a portion of a marine riser fixed to the floor of the ocean. The seal rotating with the rotating tubular allows the riser and seal housing to maintain a predetermined pressure in the system that is desirable in underbalanced drilling, gas-liquid mud systems and pressurized mud handling systems. The seal may be either an active seal or a passive seal. A flexible conduit or hose is used to compensate for relative movement of the seal housing and the floating structure.

U.S. Pat. No. 6,244,359 describes a drilling head used to seal around a drill pipe while drilling in a subsea location. The drilling head has an inner body located within an outer body. At least one bearing is located between the outer body and the inner body for facilitating the rotation of the inner body relative to the outer body. A seal mounted to a lower portion of the inner body seals around the outer surface of the drill pipe. While lowering the drilling head to the wellhead, a support attached to the drill pipe is inserted into a skirt which surrounds a portion of the seal. The skirt and support are releasably connected using a J-slot mechanism. An inner annulus and an outer annulus are located between the inner and outer bodies, the annuluses containing a lubricating fluid. Helical vanes are located within the inner annulus and affixed to the inner body. The vanes rotate with the inner body for circulating the fluid through the inner and outer annuluses.

BRIEF SUMMARY OF THE INVENTION

A dry-tree semi-submersible drilling rig comprises a drilling rotating control head attached to the top of a blowout preventer and a flexible hose attached to the pump and connected to fixed piping on the semi-submersible and in fluid communication with shakers, mud pits or diverter overboard lines at a remote location on the semi-submersible. This arrangement eliminates the need for a slip joint between the blow-out preventer and the rotary table. Elimination of the slip joint permits the drilling floor to be located at a lower elevation on the rig thereby improving the vessel's dynamics by lowering its vertical center of gravity. In certain embodiments, an in-line progressive cavity pump is attached to the rotating head.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 is a side view of a portion of a drilling vessel of the prior art having a drill floor 120 feet above deck bottom with a slip joint having a 35-foot stroke.

FIG. 2 is a side view of a drilling vessel equipped with a lowered drill floor and a mud return system according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention may best be understood by reference to the exemplary embodiment(s) illustrated in the drawing figures.

To permit lowering the drill floor of a semi-submersible, offshore drilling vessel, the present invention uses an alternative mud return system that does not require the use of a slip joint. Advantage is taken of technology that already exists in dual gradient drilling and land underbalanced drilling. In place of the slip joint and diverter under the drill floor, an industry proven drilling rotating control head is used on top of the blowout preventer. An optional, in-line progressive cavity pump that moves mud returns from the well to the shakers/mud pits may be in fluid communication with the mud return line coming off the rotating head. A flexible hose designed to the specifications for a diverter flow line may attached to the pump and connected to fixed pipe under the drill floor to direct mud returns to the shaers/mud pits or to diverter overboard lines separated by remotely-operated valves.

Advantages of the System Include:

-   -   Significantly reduced drill floor height with resulting         reduction of the rig's vertical center of gravity;     -   More convenient construction and installation of the         substructure and derrick; and,     -   Deletion of the operational requirement for removing the slip         joint during emergency evacuation procedures.

Referring now to FIG. 1, a drilling rig 10 of the prior art is depicted in side view. FIG. 1 shows a portion of a floating, offshore drilling vessel such as a semi-submersible vessel. Drilling derrick 12 is mounted on drill floor 14 and drill floor skid base 16 which, in turn, is supported on main deck 20 by support structure 18 which, in the illustrated example, is a truss-type structure.

Drilling riser 30 which extends to the wellhead on the seafloor (not shown) is connected near its upper end to riser tensioner 32. In the illustrated drilling rig, riser tensioner 32 is a ram-type tensioner and is shown in its fully extended position. Also shown in FIG. 1 are riser tensioners 32′ and 32″ in well bays to the left and right, respectively, of riser tensioner 32. The riser tensioners are shown mounted in deck box 34 below main deck 20 of the offshore vessel.

Drill string 22 (shown in FIG. 1 as a dot-dashed line) is supported by derrick 12 at its upper end and extends through diverter 38, slip joint 24, blowout preventer 28 and drilling riser 30 to the drill bit (not shown) at its lower end.

In operation, drilling fluid (“mud”) is pumped down the drill string 22 and is returned to the surface by way of the annulus between the drill string and the borehole, well casing and slip joint 24. Diverter 38 is mounted at or near the top of slip joint 24 and directs the returning mud (containing drill cuttings) to drilling fluid processing equipment on the vessel. Processed drilling fluid is stored in mud tank 36 prior to being pumped back down the drill string.

A diverter is equipment that is conventionally attached to the bell nipple on a wellhead or marine riser. The diverter can be closed to prevent fluids from flowing vertically and to divert the fluids out a blooey line, away from the rig. Two, common types of diverters are: a) a bag-type unit; and, b) a modified rotating blowout preventer. A control head, such as that described in U.S. Pat. No. 6,913,092 may be used in place of a diverter to channel drilling fluid flowing up the annulus between the drill string and the well casing to drilling fluid processing equipment on the rig.

Riser 30 is a fixed length from the seafloor. Due to vessel heave in ocean tides, swells and waves, the vessel's decks (together with the equipment mounted thereon) moves vertically relative to riser 30 and equipment attached to riser 30 such as blowout preventer 28. The maximum stroke of riser tensioner 32 is selected to accommodate the greatest heave the vessel is likely to encounter during drilling operations.

In the drilling rigs of the prior art, the changing vertical distance between the top of the blowout preventer 28 and drill floor 14 is accommodated by telescoping slip joint 24 which is guided and stabilized by roller frame module 26. Returning drilling fluid flows up the annulus between the slip joint and the drill string to diverter 38 and thence to mud processing equipment such as shale shakers and the like.

As will be appreciated by those skilled in the art, drill floor 14 must be located a sufficient vertical distance above main deck 20 to accommodate the telescoping range of slip joint 24. The higher this heavy equipment is located on the vessel, the higher the vertical center of gravity of the vessel. In general, a lower vertical center of gravity is preferable for vessel stability. Thus, improvements which permit the height of the drill floor to be lowered can favorably influence vessel stability.

An illustrative embodiment of the invention is shown in FIG. 2. Drilling rig 40 shares many components of prior art drilling rig 10 (see FIG. 1) and are correspondingly numbered.

Drill floor 14 of rig 40 is supported on main deck 20 by modified drill floor skid base 42 which has a lesser vertical dimension than support structure 16 of drilling rig 10 (FIG. 1). This lowered drill floor is made possible by an alternative mud return system which does not require a slip joint between the blowout preventer 28 and the drill floor. Drill floor 14 (and the equipment mounted thereon) may thus be lowered by up to the telescoping distance of the removed slip joint 24.

The elimination of slip joint 24 is made possible by providing diverter 44 at the upper terminus of blowout preventer 28. Diverter 44 receives returning drilling mud from the annulus between the drill string 22 and the walls of the central axial passageway of blowout preventer 28 and directs the mud via flexible conduit 48 and rigid piping 50 to on-board mud processing equipment such as shale shaker 52.

In the illustrated embodiment, shale shaker 52 is mounted on main deck 20. In other embodiments, mud processing equipment such as shale shaker 52 may be mounted in other locations such as within deck box 34. This acts to further lower the vertical center of gravity of the vessel.

Optional pump 46 may be provided to increase the return mud flow and may be necessary if mud processing equipment is located at a higher elevation on the rig. Optional pump 46 may be an in-line, progressive cavity pump.

In certain alternative embodiments, element 44 may be a control head of the type described in U.S. Pat. No. 6,913,092 containing a rotatable seal.

Although particular embodiments of the present invention have been shown and described, they are not intended to limit what this patent covers. One skilled in the art will understand that various changes and modifications may be made without departing from the scope of the present invention as literally and equivalently covered by the following claims. 

What is claimed is:
 1. A dry-tree, semi-submersible, offshore drilling vessel comprising: a support structure; a riser tensioner attached to the support structure; a drilling riser attached to the riser tensioner; a blowout preventer having a first end attached to the drilling riser and an opposing second end; and, a control head attached to the second end of the blowout preventer and having a central axial channel and a drilling fluid conduit in fluid communication with the central axial channel.
 2. The semi-submersible vessel recited in claim 1 wherein the control head is attached to the blowout preventer without the use of a slip joint.
 3. The semi-submersible vessel recited in claim 1 wherein the control head is attached to the blowout preventer with a fixed-length connector.
 4. The semi-submersible vessel recited in claim 1 wherein the control head is attached to the blowout preventer with a connector having a fixed axial dimension.
 5. The semi-submersible vessel recited in claim 1 further comprising a pump attached to the drilling fluid conduit on the rotating head.
 6. The semi-submersible vessel recited in claim 5 wherein the pump is an in-line, progressive cavity pump.
 7. The dry-tree, semi-submersible, offshore drilling vessel recited in claim 1 wherein the support structure comprises an upper deck and a lower deck and drilling fluid processing equipment is mounted on the main deck.
 8. The dry-tree, semi-submersible, offshore drilling vessel recited in claim 7 wherein the drilling fluid processing equipment comprises a shale shaker.
 9. The dry-tree, semi-submersible, offshore drilling vessel recited in claim 7 wherein the drilling fluid processing equipment comprises a mud tank.
 10. The dry-tree, semi-submersible, offshore drilling vessel recited in claim 7 wherein the drilling fluid processing equipment is installed at a lower elevation on the vessel than the elevation of the control head relative to the vessel when the tensioner is at the limit of its upward travel.
 11. The dry-tree, semi-submersible, offshore drilling vessel recited in claim 1 wherein the control head comprises a rotating seal configured to sealing engage a drill string within the central axial channel.
 12. The dry-tree, semi-submersible, offshore drilling vessel recited in claim 1 further comprising a flexible conduit having a first end that is in fluid communication with drilling fluid processing equipment located on the vessel and an opposing second end attached to the drilling fluid conduit of the control head.
 13. A dry-tree, semi-submersible, offshore drilling vessel as recited in claim 12 wherein the flexible conduit comprises an elastomer hose.
 14. A dry-tree, semi-submersible, offshore drilling vessel as recited in claim 12 wherein the flexible conduit comprises flexible steel pipe.
 15. A dry-tree, semi-submersible, offshore drilling vessel as recited in claim 12 wherein the flexible conduit comprises segments of rigid pipe connected by flexible joints.
 16. A dry-tree, semi-submersible, offshore drilling vessel as recited in claim 1 wherein the tensioner is a ram-type tensioner.
 17. A dry-tree, semi-submersible, offshore drilling vessel as recited in claim 1 wherein the support structure comprises an upper deck and a lower deck and the tensioner is mounted to the lower deck.
 18. A dry-tree, semi-submersible, offshore drilling vessel comprising: a support structure; a riser tensioner attached to the support structure; a drilling riser attached to the riser tensioner; a blowout preventer having a first end attached to the drilling riser and an opposing second end; and, a diverter attached to the second end of the blowout preventer and having a central axial channel and a drilling fluid conduit in fluid communication with the central axial channel.
 19. The semi-submersible vessel recited in claim 18 wherein the diverter is attached to the blowout preventer without the use of a slip joint.
 20. The semi-submersible vessel recited in claim 18 wherein the diverter is attached to the blowout preventer with a fixed-length connector.
 21. The semi-submersible vessel recited in claim 18 wherein the diverter is attached to the blowout preventer with a connector having a fixed axial dimension.
 22. A method for lowering the vertical center of gravity of a semi-submersible, offshore drilling vessel having a drilling floor located at a first height, a rotary table on the drilling floor, a drill string engaged by the rotary table, a drilling riser having a first, upper end supported by a riser tensioner, a blowout preventer having a first end and an opposing second end connected to the first end of the drilling riser, a slip joint having a first end connected to the first end of the blowout preventer and an opposing second end connected to a diverter [bell nipple] comprising: removing the slip joint; removing the diverter; lowering the drill floor to a second, lower height; and, installing a drilling head having a rotating seal and a drilling fluid return conduit on the second end of the blowout preventer.
 23. The method recited in claim 22 further comprising providing drilling fluid processing equipment on a lower deck of the semi-submersible in fluid communication with the drilling fluid return conduit on the drilling head.
 24. The method recited in claim 23 wherein the drilling fluid processing equipment comprises a shale shaker.
 25. The method recited in claim 23 wherein the drilling fluid processing equipment comprises a mud [drilling fluid] tank.
 26. The method recited in claim 23 wherein the drilling fluid processing equipment comprises a mud [drilling fluid] pump.
 27. The method recited in claim 23 wherein the drilling fluid processing equipment comprises at least one device from the group consisting of centrifuges, degassers, de-sanders, de-silters and hydrocyclones.
 28. A method for lowering the vertical center of gravity of a semi-submersible, offshore drilling vessel having a drilling floor located at a first height, a rotary table on the drilling floor, a drill string engaged by the rotary table, a drilling riser having a first, upper end supported by a riser tensioner, a blowout preventer having a first end and an opposing second end connected to the first end of the drilling riser, a slip joint having a first end connected to the first end of the blowout preventer and an opposing second end connected to a diverter [bell nipple] comprising: removing the slip joint; removing the diverter; lowering the drill floor to a second, lower height; and, installing a housing on the second end of the blowout preventer wherein said housing has a drilling fluid discharge conduit, an inner member rotatable relative to said housing and having a passage through which the drill string may extend, and a seal moving with said inner member to sealingly engage the drill string. 